Fix ODS Reader when no DC namespace are defined

[Jerome3]

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

ODS files without spreadsheet properties were triggering a fatal error Issue #1176.

[rtista]

Can someone please check if the correction is valid and accept the merge request? This happens for a lot of ODS files used nowadays..

[PowerKiKi]

Thanks for the PR !